PARTITIONING OF MODERATELY SIDEROPHILE ELEMENTS AMONG OLIVINE, SILICATE MELT, AND SULFIDE MELT: CONSTRAINTS ON CORE FORMATION IN THE EARTH AND MARS

Abstract

This study investigates the effects of variations in the fugacities of oxygen and sulfur on the partitioning of first series transition metals (V, Cr, Mn, Fe, Co, Ni, and Cu) and W among coexisting sulfide melt, silicate melt, and olivine. Experiments were performed at 1 atm pressure, 1350°C, with the fugacities of oxygen and sulfur controlled by mixing CO2, CO, and SO2 gases. Starting compositions consisted of a CaO-MgO-Al2O3-SiO2-FeO-Na2O analog for a barred olivine chondrule from an ordinary chondrite and a synthetic komatiite. The fo2/fs2 conditions ranged from log fo2 = −7.9 to −10.6, wi log fs2 values ranging from −1.0 to −2.5. Our experimental results demonstrate that the fo2/fs2 dependencies of sulfide melt/silicate melt partition coefficients for the first series transition metals are proportional to their valence states. The fo2/fs2, dependencies for the partitioning of Fe, Co, Ni, and Cu are weaker than predicted on the basis of their valence states. Variations in fo2/fs2 conditions have no significant effect on olivine/melt partitioning other than those resulting from fo2-induced changes in the valence state of a given element. The strong fo2/fs2, dependence for the olivine/silicate melt partitioning of V is attributable to a change of valence state, from 4+ to 3+, with decreasing fo2. Our experimentally determined partition coefficients are used to develop models for the segregation of sulfide and metal from the silicate portion of the early Earth and the Shergottite parent body (Mars). We find that the influence of S is not sufficient to explain the overabundance of siderophile and chalcophile elements that remained in the mantle of the Earth following core formation. Important constraints on core formation in Mars are provided by our experimental determination of the partitioning of Cu between silicate and sulfide melts. When combined with existing estimates for siderophile element abundances in the Martian mantle and a mass balance constraint from Fe, the experiments allow a determination of the mass of the Martian core (~17 to 22 wt% of the planet) and its S content (~0.4 wt%). These modeling results indicate that Mars is depleted in S, and that its core is solid.